This invention relates to polymer clay nanocomposites, and more particularly to methods of measuring properties of such composites including devices especially designed to practice such methods.
Polymer clay nanocomposites are formed when nano-sized clay additives are dispersed into a plastic. The dispersion of nanoscopic particles in polymers can provide materials properties that can not easily be achieved by the incorporation of conventional macroscopic fillers. Nanocomposites based on organically modified smectite clays (organoclays) have received significant attention over the last decade but unmodified clays have also been incorporated in plastics. Examples of important applications for these clay and organoclay nanocomposites are in the area of flame retardancy, barrier enhancement, composite strength, and improvements in thermal stability. Elementis Specialties, Inc., assignee hereof, is the world's largest manufacturer of organoclays and is a recognized leader in research and development relating to this field.
Hydrous phyllosilicate clays are particularly useful for the preparation of polymer organoclay nanocomposites. These hydrous phyllosilicate clays are layered, platy materials where the individual clay layers are aggregated into discrete particles. Useful phyllosilicate clays include smectite clays such as: montmorillonite; bentonite; hectorite; saponite; stevensite; and beidellite, for example. Other useful phyllosilicate clays are vermiculite and certain micas. Hectorite, montmorillonite and bentonite are particularly useful. Both naturally occurring- or synthetic-clays, particularly synthetic hectorite, may be used in the preparation of polymer clay nanocomposites. When the word clay is used hereafter, it includes both natural and synthetic clays.
Clay layers are hydrophilic because they carry a net negative charge that is neutralized by hydrophilic metal ions that are positioned between the surfaces of the clay layers. Organic surface treatment is often necessary to produce modified clays called organoclays that have improved compatibility with organic systems. An organoclay is formed when the hydrophilic metal ions are exchanged with organic cations. Such organoclays are easily prepared and are readily available as discussed hereafter.
Polymer organoclay nanocomposite benefits are realized when one or more organoclays is delaminated to the “nano” state in a polymer. It is generally accepted that, ideally, all organoclay particles are dispersed towards individual layers in a delamination process to form a polymer composite containing discrete nano-sized layers. This “delamination of layers” is at times also referred to as “exfoliation of layers” and both terms can be used interchangeably. However, complete dispersion of these organoclays towards their nano-sized state is not easily accomplished. Incomplete organoclay dispersion and delamination leaves microsized particles in the plastic which may alter the materials properties of the resulting nanocomposite. The degree of organoclay dispersion in plastics is extremely difficult to control and such inability can result in unpredictable materials properties. Clay nano-dispersion repeatability and reproducibility has been a long-felt need in the field of clay nanocomposites. If such repeatability and reproducibility could be achieved, polymer organoclay nanocomposites would likely become much more successful in the marketplace. Currently there is no facile analytical method available to quantify the degree of organoclay delamination in plastics. Transmission electron microscopy (TEM), X-ray Diffraction (XRD) and Thermogravimetric analysis (TGA) are widely used to characterize polymer organoclay nanocomposites, but none of these methods can effectively quantify the degree of organoclay delamination; they merely provide a rough idea about the clay dispersion state. Furthermore, certain applications benefit when the delaminated layers are arranged in an aligned state. Nanocomposites that are targeted to deliver enhanced barrier properties preferably have exfoliated and aligned clay platelets. Analysis of clay layer alignment alone is not meaningful for such composites as the clay platelets could be aligned while still being stacked in particles or agglomerates. Layer alignment is only meaningful for such composite materials when interpreted in conjunction with the degree of layer delamination. Furthermore, measurement of degree of delamination alone is not meaningful for these nanocomposite as the application depends on the presence of delaminated and aligned clay layers.
A general introduction to assess materials orientation via the aid of polarized vibrational spectroscopy is discussed by Everall in, “Using polarized vibrational spectroscopy to characterize molecular orientation in polymers: An introduction—a tutorial,” The Internet Journal of Vibrational Spectroscopy, 3(2), 1998, where the author disclosed using vibrational spectroscopy to quantify crystalline and amorphous phase orientation in polyethylene.
For background on clays and spectroscopy, also see Schaefer, et al., “Extent of dispersion of an organo-clay complex in oil. An infrared method,” NLGI Spokesman, 34, 418 (1971), which discloses that the silicon-oxygen (Si—O) bond infrared (IR) absorption bandwidth reduces when organoclay is dispersed in mineral oil to produce grease.
Note that the clay layer orientation has been determined for a Nylon 6-clay nanocomposite film using a combination of infrared and TEM analysis, as disclosed by Loo, et al., in “Investigation Of Polymer And Nanoclay Orientation Distribution In Nylon6/Montmorillonite Nanocomposite,” Polymer, 45, 5933 (2004).
A quick and easy to use analytical method that is able to measure the degree of organoclay delamination and layer alignment in a polymer-organoclay nanocomposite is very desirable. The above mentioned techniques have not been shown to address this issue. With the aid of an analytical method that can assess clay layer dispersion and orientation, polymer organoclay nanocomposite producers could guarantee the quality and consistency of their polymer composite products.